The present invention relates to a process for treating spent electroless nickel plating solutions to remove objectionable contaminants from the solution such as nickel and phosphorus species. The process of this invention represents a low cost alternative to present technology which frequently requires off-site disposal to satisfy existing and projected stricter environmental standards.
Electroless nickel plating is a popular commercial technique for depositing nickel coatings on a suitably treated surface by the controlled chemical reduction of nickel ions. The nickel coating catalyzes the reduction reaction, and the deposition of nickel continues as long as the substrate remains in contact with the electroless nickel plating solution. Typical applications of electroless nickel coatings include parts which are difficult to plate such as valves and machine tools, as well as larger objects such as caustic railcars and barges. The physical properties of the coated surface such as coating uniformity, corrosion and wear resistance, lubricity and ductility compare favorably with electroplated nickel surfaces and are more economical for industrial applications.
Electroless nickel plating baths are mixtures of several chemicals each performing specific functions. A source of nickel ions, such as nickel chloride or sulfate is required. Additional components include reducing agents to supply electrons for the reduction of free nickel ions, complexing agents to control the amount of free nickel ions in solution, buffering agents to resist the pH changes associated with the nickel reduction reaction, accelerators to enhance the speed of the reaction, and inhibitors to moderate the deposition process. The electroless nickel plating bath is normally maintained at a temperature in the range of from about 85.degree. C. to about about 95.degree. C.
Typical ingredients of electroless nickel plating solutions include, as a reducing agent, sodium hypophosphite, complexing and/or buffering agents such as citric acid, acetic acid, hydroxyacetic acid, succinic acid, lactic acid, malic acid, propionic acid, and aminoacetic acids, accelerators such as succinic acid, and inhibitors such as thiourea or lead nitrate.
The major types of wastewater resulting from electroless nickel process are spent electroless nickel plating solutions, stripping solutions, and rinse waters. Stripping solutions employ nitric acid to remove nickel from improperly plated surfaces and to remove nickel deposits from the surfaces of plating equipment. The spent plating solutions contain relatively large amounts of total soluble nickel species (Ni.sup.+2), as well as reducing and complexing agents.
The on-site treatment of spent electroless nickel plating solutions is required to meet effluent discharge limits which can be as low as 1 mg/l of nickel (Ni.sup.+2) and/or total phosphorus. In practice, this is difficult to achieve due to the presence of high concentrations of complexing agents in the spent solution. Off-site disposal is very expensive, and this expense frequently renders this alternative economically unfeasible.
Existing treatment methods for removing nickel species from industrial waste waters generally involve either the reduction of soluble nickel to elemental nickel, the precipitation of insoluble nickel compounds, or the separation of the nickel by adsorption, electrostatic force, applied electrical potential, and hydraulic or mechanical pressure. The precipitation of ionic nickel as nickel hydroxide is frequently the most convenient and/or least expensive alternative.
Typical spent electroless nickel plating solutions also contain high concentrations of hypophosphite and phosphite species. As used in this specification and claims, the term "hypophosphite" refers to the concentration of those phosphorus species in the +1 oxidation state, the term "phosphite" refers to the concentration of phosphorus species in the +3 oxidation state, and "phosphate" refers to the concentration of phosphorus species in the +5 oxidation state.
The phosphite species are produced by the oxidation of sodium hypophosphite when soluble nickel is reduced to elemental nickel in the electroless nickel plating process. Although hypophosphite species, and to a lesser extent phosphite species, are soluble in water, phosphate species can be removed by precipitation using a suitable precipitating agent such as lime, ferric chloride, or alum to produce insoluble metal phosphate species. Lime is the preferred precipitating agent.
The one-step methods for removing hypophosphite and phosphite species from spent electroless nickel plating solutions are either ineffective, i.e. such as the use of lime precipitation, or too costly, i.e. by using ion exchange methods for instance.
An effective treatment process for spent electroless nickel plating solutions must also address, in addition to economics, the problem of treating the wide variety of chemicals encountered in typical commercial electroless nickel plating baths.